What are Hag-fishes and their Evolution?

Introduction:

Hag-fish belong to the class Myxini, which are also known as Hyperotret and order Myxiniformes. These are eel-shaped and slime-producing marine fishes. Sometimes they are also called slime eels. These are the only known living craniates that possess a skull but no vertebral column. Hag-fish have rudimentary vertebrae. Lampreys and hag-fishes are jaw-less. Living hag-fish remain identical to hag-fish from almost 300 million years ago.

 

Hag-fishes are scaleless and soft-skinned animals having paired and hard and thick barbels on the end of the snout. Different species, grow to about 40 cm to 100 cm long in length. Primitive hag-fishes possess a tail fin, but no paired fins, and also no jaws or bones were present in them. Skeletons are made up of cartilages, having mouths that are round and slit-like openings with horny teeth. They have poorly developed eyes buried under the skin, and they have one nostril at the end of the snout. They use 5 to 15 pairs of gills for respiration. The pairs of gills share the same opening on each side in all the members of the family Myxinidae but open separately on the surface like the family Eptatretidae.

 

Hag-fishes are found in cold seawater, to depths of about 1,300 m. They live on soft bottoms mostly in burrows and habitually lie buried in, their whole body except for the tip of the head. Their diet comprises marine invertebrates and dead and crippled fishes. Sometimes, hag-fishes attack fish caught on lines or in nets, boring their way into the bodies and eating the fish from the inside. To prevent predators, hag-fish have fine pores along the body that secrete copious amounts of slime. In this way, they protect themselves from attacks by animals.

 Classification:

Kingdom: Animalia

Phylum: Chordata

Class: Myxini

Order: Myxinifomes

Family: Myxinidae

Evolution:

Paleontologists discovered the first complicated fossil of a hag-fish which are the slimy and eel-like carrion feeders organisms of the ocean. The 100-million-year-old fossil helps us answer questions about when these ancient and jaw-less fish branched off the evolutionary tree from the lineage which gave rise to modern-day jawed vertebrates and craniates including all bony fish and also humans.

 One fossil, named “Tethymyxine tapirostrum”, is a 12-inch long fish that was embedded in a slab of Cretaceous period limestone, found in Lebanon. It completes a gap of 100-million-year in the fossil record and helps to determine that hag-fish are more closely related to the blood-sucking lamprey than to other fishes. It means that both hag-fish and lampreys evolved their eel-like body shape and anomalous feeding systems after they branched off the evolutionary tree from the other vertebrate line of origin almost 500 million years ago.

 

Characteristics of the new fossil help to place the hag-fish and their other relatives in the family of vertebrates. In the past, scientists have conflicted about where they belonged and relied on how they tackled the question. Those who depend on fossil evidence alone supervise to conclude that hag-fish are so primitive that they are not only even vertebrates. This hypothesis indicates that all fishes and their vertebrate descendants share a common ancestor that more or less looked like hag-fish.

But some other scientists who work with genetic data of hag-fish argue that hag-fish and lampreys are more closely related to each other. This indicates that modern hag-fish and lampreys are the strange ones out in the family tree of vertebrates. In that case, the primitive appearance of hag-fish and lampreys is deceitful and proposes the hypothesis that the common ancestor of all vertebrates was probably something more conventionally fish-like.